I was lucky enough to end up with a Redfield accelerator reflex sight. I was thinking about putting it on my M&p15 as a 45 degree backup sight to my 3-9 scope.

But I want to consider putting it on the M&P9. As far as I know, there are two options. One is to replace the rear sight with an adapter plate, which is pretty inexpensive. The other is to have the slide machined to directly mount the sight, which keeps things lower and allows iron sights to co-witness.

I'm looking for insights on this, potential sources for machining or adapter plates, or horror stories about how it ruined the pistol. And advice is appreciated.

The Redfield instruction manual says that it is compatible with JPoint and FastFire mounting plates. If this is the case, you should be able to get one direct from Burris or through Speed Shooter Specialities.

S&W M&P : Welcome to Speed Shooter Specialties

Every top USPSA and Steel Challenge shooter I personally know, uses a bridge mount and do not mount an optic directly to the slide.
It seems that no matter what the manufacturer claims, they don't hold up long term, either by breaking down or losing POA. With a lot of use and the beating they take, when mounted to the slide, it seems inevitable. I have two rimfires for Steel Challenge; an M&P22 for limited (iron sights) and a Buckmark with a C-More RTS2 for the open class.

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This is great! Thanks!

For a bridge-type mount that attached to the rail under the barrel, search for "Mako universal picatinny mount"